Alkali metal vapor lamp manufacture



May 11, 1965 K. scum/um 3,183,051 ALKALI METAL VAPOR LAMP IANUPAdTURE Filed larch l, 1963 Inven=tof-g f Kurd,- S

AL KALI METAL VAPOR LAMP MANUFACTURE Kurt Schmidt, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Filed Mar. 1, 1963, Ser. No. 262,107

3 Claims. (Cl. 316-3) entitled "Sodium Vapor Lamp and assigned to the same assignee as the present invention. Since glass and even quartz cannot withstand: the attack of sodium vapor at of sintered transparent polycrystalline alumina. This ma- United States Patent ..20 highltemperatures, that lamp utilizes a tubular envelope terial has a very high alumina content, for instance in excess of 99.5 percent A1 0 has exceedingly good light transmittance, is impervious to sodium, and can withstand the attack of its vapor without blackening for long periods of; time even at temperatures as high as 1600 C. The lamp contains a filling of sodium and an inert gas, preferably xenon. v I

Since sodium is highly reactive, it must not be exposed to air or oxygen during handling. Upon exposure to air, it fumes and bursts into flame; Since it is also reactive with water, the conventional laboratory procedure is to keep sodium iiroil; this is not practical for lamp manufacture and in any case it'contaminates the sodium.

In one type of high pressure sodium vapor lamp, which may be referred to as the saturated vapor type, an excess amount of sodium is placed in the envelope and the desired'sodium vapor pressure during operation is obtained by controlling the temperature of a suitable part of the lamp, for instance the metal exhaust tube appendix. In another type of high pressure sodium vapor lamp, the unsaturated vapor type, a limited amount of sodium is placed in the envelope and vaporizes completely during operation to give the desired lamp characteristics, In such case, the quantity of sodium must be accurately controlled, thus compounding the handling problem.

The object of the invention is to provide a new, convenient, and practical method of introducing an alkali metal such as sodium into an electric lamp envelope. A desirable feature of the method is that it can be utilized with a polycrystalline alumina ceramic envelope.

Another object of the invention is to provide a method of introducing sodium into a lamp envelope which allows the quantity'introduced tobe readily determined and controlled.

According to the invention, a measured quantity of alkali metal is introduced into a sealed lamp or device by electrolyzing the metal from a molten bath of a salt of the metal through a vitreous wall or partition attached to the lamp or device. The vitreous wall is of such nature that it is conductive at the temperature of the molten salt bath by virtue of high mobility of the ions of the alkali metal which is present as one of its constituents.

In a preferred method for introducing a'measured quantity of sodium into an alumina ceramic lamp, the lamp is constructed in the form in which it is finally intended,

but with a glass tube attached to it through a metal exhaust tube. In this form, the lamp is processed to the stage where the electrodes are activated and the inert gas filling is introduced, the glass tube appendix being ice tipped-off. The glass appendix is then immersed in a molten sodium salt bath at a temperature sufiiciently high to make the glass conductive. Direct current is then passed through the salt bath into the lamp,'current flow being sustained by electrolysis through the bath and by an electric discharge through the appendix. The glass is conductive by virtue of the high sodium ion mobility therein at high temperatures and the discharg'ecurrent is maintained by the flow of sodium ions through the wall of the appendix. The sodium is effectively electrolyzed into the glass appendix and the amount transported is readily calculated through application of Faradays law, the quantity being directly proportional to the product of current by time. The sodium is then transferred from the glass appendix into the alumina ceramic tube by heating the appendix and cooling the tube. The metal exhaust tube is then pinched shut to seal off the ceramic tube; the ceramic tube is now complete and the glass appendix may be discarded.

'For further advantages and features of the invention, attention is, now directed to the following description of a preferred process embodying the invention, to be read in conjunction with the accompanying drawing. The

features of the invention believed to be novel'will be more particularly pointed out in the appended claims.-

In the drawing, wherein like symbols denote corresponding parts in both figures:

FIG.. 1 illustrates an alumina ceramic lamp with glass appendix attached, arranged in a set-up, shown in partly diagrammatic form, for electrolyzing sodium into the appendix.

FIG- 2 similarly illustrates the set-up for driving the sodium from the glass appendix into the ceramic tube or lamp.

As illustrated in FIG. 1, a typical high pressure sodium alumina lamp 1 towards which my process is more particularly directed, comprises an envelope 2 of ceramic tubing of sintered transparent polycrystalline alumina. The ceramic may be prepared in the manner described in Patent 3,026,210-Coble, Transparent Alumina and Method of Preparation. By way of dimensional example, the. tubing may be 8.5 centimeters in over-all length by 6 millimeters in internal diameter, the wall thickness being approximately 0.8 millimeter except forthe belled ends where it is increased to about 1.4 millimeters for greater strength. The end closures comprise thimbles or cap-like members 3, 4 of niobium which fit into the ends of the tube with suflicient radial clearance to allow capillary flow of a glassy sealing material between the mating surfaces. A suitable glassy material consists of a eutectic or near-eutectic mixture comprising mainly aluminum oxide and calcium oxide and having a .flow point or melting point above 1400 C. Metal exhaust tubes 5, 6, likewise suitably of niobium, penetrate into the thimbles and are welded to them. The electrodes proper 7, 8 each comprise a rod or stud of thoriated tungsten which is crimped or welded at one end into the exhaust tube and has a tungsten coil slipped or screwed over its other end. The tungsten coils are coated with activating material which is also accommodated in the interstices between turns. The upper exhaust tube 5 is pinched 011 as by a cold weld indicated. at 9. The lower exhaust tube 6 is provided with a lateral aperture at 10 just behind the crimp to the electrode rod but forward of the weld to the thimble. This provides a passageway through which the ceramic tube may be exhausted and a filling introduced. In general, the ceramic lamp and particularly the attachment of the end closures and electrodes, may be constructed in the manner described in copending application Serial No. 247,583 filed December 27, 1962 of William C. Louden and Richard S. Pinter, entitled Ceramic Lamp Construction, and assigned to the same assignee as the present invention.

To complete the lamp in accordance with the present invention, a glass appendix 11 is first attached in such manner as to be in communication with the ceramic envelope or tube. The glass appendix is preferably made of a hard borosilicate glass (Pyrex), such glass containing sodium as one of its constituents. It is attached by means of a Kovar metal tube 12 which is sealed at one end to a graded sealing glass section 13 joined to the glass appendix and welded to the niobium exhaust tube 6 at the other. Kovar is an alloy of iron, nickel and cobalt having a coefficient of expansion closely matching that of the sealing glass section so that a hermetic seal may readily be made thereto. In FIG. 1, glass appendix 11 is shown with its lower end tipped ofli at 14 but initially a tube-like extension is provided at the end through which connection may be made to a gas exhausting and filling system. After the glass appendix is attached, the lamp is processed in the usual fashion but leaving out the introduction of the sodium filling. In other words, it is exhausted and baked; the electrodes are activated while still under exhaust; and a filling of an inert starting gas, suitably xenon, is provided. The lamp and glass appendix combination is then tipped-off at 14 to provide the hermetically sealed structure illustrated in FIG. 1.

The next stage consists in electrolyzing sodium into the glass appendix. To this end, the glass appendix is immersed in a molten sodium salt bath indicated at 15 within a quartz or hard glass beaker 16. The beaker is surrounded by an electric resistance heating element 17, to which current from the usual 115-120 volt A.-C. supply may be controlled by a switch 18 in order to maintain the salt bath at the desired temperature. Conveniently, the switch may be of a thermostatic type and insulation may be provided to enclose the beaker and heating element. The sodium salt must be one that liquifies at a temperature between approximately 200 and 500 C. and which readily electrolyzes without at the same time liberating dangerous gases. Sodium nitrate is suitable; other materials which may be used are sodium nitrite, sodium oleate and other sodium salts having a melting point within the desired range. The ceramic lamp 1 is cooled by a water-bath, being for this purpose inserted into a glass tube 19 through which water is circulated. The main purpose of cooling the lamp is to prevent oxidation of the end closures and niobium exhaust tubes and to this end, the major part of the lower exhaust tube 6 projects into the glass tube through the soft rubber stopper 20 closing its lower end.

A direct current potential at least sufficiently high to maintain a discharge within the glass appendix is now applied between the metal tubes joining the glass appendix to the ceramic lamp and an electrode 21 of non-reactive material, suitably a platinum or stainless steel plate or a carbon rod, which is immersed in the salt bath. Alternatively, the negative voltage may be applied to electrode 7 at the other end of the lamp or to some other conductive member able to serve at least temporarily as cathode. I have found a potential of 1500 volts D.-C. generally adequate, a resistance 22 being provided in series to limit the current to a practical figure such as 100 milliamperes. A high frequency pulse generating device or sparking coil may be used momentarily to strike the are. Applying the D.-C. potential with the polarity indicated in the drawing the metal tube operates as cathode and electrode 21 operates as anode. At the relatively high temperature of the sodium salt bath, the glass of the appendix is conductive by virtue of high sodium ion mobility. In other words, the anode is translated to the walls of the glass appendix and an electric discharge is established within the appendix through the filling gas (xenon). The current carriers through the salt bath are the sodium ions which are carried right through the glass wall. They are thus so to speak electrolyzed into the appendix where the positive charge on the sodium ions is neutralized by electrons of the discharge. The neutralized sodium atoms remain as a vapor or as a solid as the pressure builds up within the glass appendix. The amount of sodium transported through the glass is proportional to the product of current by time and is readily calculated through application of Faradays law.

Where it is desired to provide some other alkali metal filling, a glass is selected for the appendix which contains the desired alkali metal as one of its constituents. For instance, a potassium or cesium containing glass is used where a potassium or cesium filling is desired. Of course, an appropriate salt of the desired alkali metal is then selected for the molten salt bath.

The final stage consists in shifting the sodium and xenon from the glass appendix to the ceramic lamp envelope. This is accomplished by applying sufficient heat to the glass appendix to vaporize the sodium, and chilling the far end of the ceramic lamp sufficiently to condense and collect the xenon therein. An arrangement such as illustrated in FIG. 2 may be used wherein the glass appendix is inserted uppermost into a hard glass tube 23 with only a small portion of the ceramic tube projecting out through a stopper 24 at the lower end. Tube 23 is heated by an electric resistance element 25 coiled around it to which current flow from the usual -120 volt A.-C. source is controlled by a switch 26 which may be of a thermostatic type in order to maintain a constant temperature. An inactive or inert gas, suitably argon is caused to flow into tube 23 through inlet 27 and out through a constricted outlet 28. The argon atmosphere within tube 23 prevents oxidation of the niobium thimble 4 and exhaust tube 6. The exposed end of the ceramic lamp, or at any rate thimble 3 and exhaust tube 5, are immersed in a beaker 29 filled with liquid nitrogen 30. The heat applied to the glass appendix and to the connecting metal tubes vaporizes the sodium which diffuses to the chilled end of the ceramic tube where it condenses.

Likewise the xenon is condensed into a liquid or solid and collects at the chilled lowermost end of the ceramic tube or lamp. The lamp and appendix assembly is then removed and exhaust tube 6 pinched and severed in order to seal off the ceramic lamp, the glass appendix being then discarded. The completed lamp now contains substantiall the entire quantity of sodium which was electrolyzed into the appendix.

The specific apparatus and set-up which has been illustrated and described to carry out my method inclines to be of a laboratory nature and is intended by way of example only. Various modifications will readily occur to those skilled in the art for adapting the process to factory conditions, and it is intended by the appended claims to cover any such falling within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The method of introducing a charge of alkali metal into a sealed electric device impervious thereto which comprises attaching a glass appendix to said device through a conductive joining tube capable of serving as an electrode, the glass of said appendix containing said alkali metal as a constituent, introducing a filling of ionizable gas into the combination of device and appendix and sealing off the combination, immersing the appendix into a molten bath of a salt of said alkali metal maintained at a temperature where the glass is conductive by reason of alkali ion mobility, applying a direct current potential in negative polarity to said conductive joining tube and in position polarity to an electrode immersed into said salt bath, said potential being sutficiently high to maintain a discharge in said glass appendix between said conductive joining tube and the walls of said appendix whereby alkali metal ions are electrolyzed directly through the walls of said appendix, transferring the alkali metal from the appendix into the device by applying "neat to the off the combination, immersing the appendix into a molten bath of a sodium salt maintained at a temperature where the glass, isconductive by reason of sodium ion mobility, applying a direct current potential in negative polarity to said conductive joining tube and in positive polarity to an electrode immersed into said salt bath, said potential being sufficiently high to maintain a discharge in said glass appendix between said conductive joining tube and the walls of said appendix whereby sodium ions are electrolyzed directly through the walls of said appendix, transferring the sodium from the appendix into the device by applying heat to the appendix and cooling the device,

and thereafter severing and sealing the connection between said device and the glass appendix to tip-off the device.-

'3. The method of introducing a measured charge of sodium into an alumina ceramic lamp of the kind having a metal exhaust tube which comprises attaching a glass appendix to said metal exhaust tube in communication with said ceramic lamp, processing the lamp with the glass appendix attached including providing an inert gas filling therein and tipping-off the appendix, immersing the appendix into a molten sodium salt bath maintained at a temperature in the range of 200 to 500 C., electrolyzing sodium from said molten salt into said appendix through its glass walls by causing current to flow from an electrode immersed into said molten salt bath, through the salt bath and glass wall of the appendix by electrolysis, and through the appendix -by electric discharge in the inert gas filling to said metal exhaust tube, transferring the sodium from the appendix into the ceramic lamp by applying sufficient heat to the appendix to vaporize the sodium and cooling the lamp to condense the sodium therein, and

pinching-off said metal exhaust tube in order to seal the ceramic lamp and sever off the appendix.

References Cited by the Examiner UNITED STATES PATENTS 1/26 Ruggles 316-3 5/26 Van Voorhis 316-3 XR OTHER REFERENCES Journal Optical Society of America, volume '11, pages 8'], to 91, July 1925. Sodium by Electrolysis through Glass by Robert C. Burt.

FRANK E. BAILEY, Primary Examiner. 

1. THE METHOD OF INTRODUCING A CHARGE OF ALKALI METAL INTO A SEALED ELECTRIC DEVICE IMPERVIOUS THERETO WHICH COMPRISES ATTACHING A GLASS APPENDIX TO SAID DEVICE THROUGH A CONDUCTIVE JOINING TUBE CAPABLE OF SERVING AS AN ELECTRODE, THE GLASS OF SAID APPENDIX CONTAINING SAID ALKALI METAL AS A CONSTITUENT, INTRODUCING A FILLING OF IONIZABLE GAS INTO THE COMBINATION OF DEVICE AND APPENDIX AND SEALING OFF THE COMBINATION, IMMERSING THE APPENDIX INTO A MOLTEN BATH OF A SALT OF SAID ALKALI METAL MAINTAINED AT A TEMPERATURE WHERE THE GLASS IS CONDUCTIVE BY REASON OF ALKALI ION MOBILITY, APPLYING A DIRECT CURRENT POTENTIAL IN NEGATIVE POLARITY TO SAID CONDUCTIVE JOINING TUBE AND IN POSITION POLARITY TO AN ELECTRODE IMMERSED INTO SAID SALT BATH, SAID POTENTIAL BEING SUFFICIENTLY HIGH TO MAINTAIN A DISCHARGE IN SAID GLASS APPENDIX BETWEEN SAID CONDUCTIVE JOINING TUBE AND THE WALLS OF SAID APPENDIX WHEREBY ALKALI METAL IONS ARE ELECTROLYZED DIRECTLY THROUGH THE WALLS OF SAID APPENDIX, TRANSFERRING THE ALKALI METAL FROM THE APPENDIX INTO THE DEVICE BY APPLYING HEAT TO THE 